According to German Patent No. 1 004 516, the lens is for this purpose clamped at high pressure between two cups, so that its position cannot change by itself. To center the lens, the clamping cups are vibrated by ultrasound during the clamping operation in order to convert the static friction between cup and lens into a lower sliding friction. However, this transition occurred sporadically, which often caused damage to the lens due to an undesired material removal.
Furthermore, an attempt has been made to drive the clamping cups during clamping of the lens in opposite directions of rotation Here too a high risk exists that lens damage will occur, that is, cutting tracks in the form of rings cut into the surface of the lens can hardly be avoided.
German Auslegeschrift No. 21 48 102 suggests to arrange a piezoceramic case vibrator on the elevationally nonchangeable clamping cup, which case vibrator is electrically controlled by a threshold switch such that the clamping cup force drops off when reaching a given pressure, which causes the vibration generator to be turned off. The piezovibrator is used at the same time to test the clamping pressure, to which the vibrational amplitude is regulated. Electronic instabilities are disadvantageous in this arrangement. Furthermore, the vibrator has a not insignificant sensitivity with respect to axial pressure. An initial stress is created during clamping due to the pressure load; a supporting of the vibrator is therefore problematic.
From German Offenlegungsschrift No. 31 39 873 a device is known in which the irregularities of a gear drive are utilized to produce relative movements between lens and clamping cup. A balanced differential is provided as a compensating device between the two parts of a two-part centering spindle and the drive shaft. A hydraulic clamping cylinder is provided for a pressure plate of the upper, axially movable spindle. Due to the high friction of the clamping spindle in its slide bearing, a precise regulating of the clamping pressure is, however, difficult to realize, so that this device can also only be utilized in a limited way.
The purpose of the invention is, while overcoming the disadvantages of the state of the art, to improve the centering of the bearing support and clamping of optic lenses in an economical way such that the adjustments for the cutting operation and the cutting operation itself can be carried out in a short period of time, easily and precisely with the least possible pressure load on the lens. By providing, at least for the lower centering spindle, a guide bearing loadable with air, the invention achieves in an extremely simple manner a transition to a sliding friction during an axial movement of the centering spindle, the frictional forces being thereby reduced to approximately one tenth of the static friction forces. The very smoothly responding arrangement makes it possible for the entire centering operation to occur during a phase where light contact with the lens exists.
It is also possible to provide an air bearing for the upper centering spindle.
Each air bearing has a thin guide sleeve which snugly encloses the respective centering spindle or rather its outer sleeve and itself is surrounded by a pressure-loadable cavity. This arrangement is structurally simple and permits one to adjust and/or regulate the clamping pressure as needed in order to clamp spindles for the machining task.
The clamping bearings are arranged aligned in closed chambers of the housing. The housing is, due to a honeycomb-shaped construction having a generally C-shaped design, resistant to twisting, so that the clamping bearings maintain their alignment even during a high load.
The pressure can in the upper clamping bearing be adjustable or resettable by means of an adjusting device. At least for the lower clamping bearing there is provided, a connection to the pressure-medium supply with a controllable or adjustable pressure. The desired pressure relationships can in this manner be created clearly and precisely.
An important further development of the invention, consists in the oppositely lying air bearings being reciprocally loadable with a higher and lower pressure. The frequency and pressure of the air load can thereby be regulatable or rather adjustable pneumatically and/or electrically. The clamping spindle is thus vibrated in its air bearing by placing two oppositely arranged air cushions reciprocally under increased pressure. The vibrations can be produced in a conventional manner by a vibrator. The clamping cup and the lens is also moved translatorically by the translatoric vibration introduced through the air cushion on the clamping spindle. Since the lens is not fixedly connected to the clamping cup and rests on it only by its own weight, small sliding movements between the lens and clamping cup are created by its mass moment of inertia. This changes the static friction to a sliding friction and thus the friction coefficient is also reduced. If the moving lens during alignment touches the upper clamping cup, the aligning forces are also significantly reduced by the reduced friction coefficient. The minimum clamping angle of 16.degree. fixed by the static friction can thus be substantially reduced and the area of the self-centering lens can be enlarged. The air bearings can be especially designed so that the inner wall or rather each guide sleeve has channels and/or pockets, in particular in the form of four separate air-cushion fields. A further specialization provides that the channels or rather pockets are constructed as axially parallel and/or partial-ring-shaped grooves. The very simple construction guarantees a precise guiding with the least possible friction during a loading with pressurized air, which is also advantageous during the cutting operation in order to keep cooling and lubricating means and abrasive material away from the spindle bearing